That it is not "tenable" is your opinion. As I said, and Msgr. Fenton's article demonstrates, it is still an open question. Or would one be a "heretic" according to Lad to hold it? 
Msgr. Fenton notes of the contrary opinion - i.e., that occult heretic's are not members - that it is "still the unsubdued opposition to St. Robert's thesis." See page 216 (emphasis added).
Stubborn, you might like to read page 209, where the view of Cajetan and others that "all baptized persons" are "parts or members of the true Church," and "that the baptismal character constituted even a public apostate or heretic a genuine member of the Church," is mentioned. He does say that Cajetan's opinion did not survive the Counter-Reformation period. Page 213.
The first part, that says: "all baptized persons are parts or members of the true Church," is incomplete, therefore not the whole truth which results in being cause for confusion.
Very simply, baptized prot babies are members, but only until the age of reason. If they never become Catholic after the age of reason, then they remain outside of the Church unless or until they do become Catholic.
The bolded is only true if apostates / heretics had the Catholic faith before falling into the sin of heresy.
The distinction is, *having the Catholic faith*, because this is what makes one Catholic. To lose the faith by whatever means is itself a sin because to lose the faith is to not believe in the Church, which is Christ, see John 16:9 (" And when he is come, he will convict the world of sin, and of justice, and of judgment. [9]
Of sin: because they believed not in me").